core: change logging from environment variable to configuration file-based

GlobalConfigurationFixture uses unit-tests.conf to determine default
and module specific logging levels. Logging will gracefully default to
INFO if unit-tests.conf is not found. See unit-tests.conf.sample for a
sample configuration file.

refs: #1375, #1267

Change-Id: Ib0c4eb4149748e6658f94ef1afa23ddd3072c0fa
diff --git a/nfd.conf.sample.in b/nfd.conf.sample.in
index 3cbfc0f..b825cca 100644
--- a/nfd.conf.sample.in
+++ b/nfd.conf.sample.in
@@ -3,6 +3,38 @@
 ; {
 ; }
 
+log
+{
+  ; default_level specifies the logging level for modules
+  ; that are not explicitly named. All debugging levels
+  ; listed above the selected value are enabled.
+  ;
+  ; Valid values:
+  ;
+  ;  NONE ; no messages
+  ;  ERROR ; error messages
+  ;  WARN ; warning messages
+  ;  INFO ; informational messages (default)
+  ;  DEBUG ; debugging messages
+  ;  TRACE ; trace messages (most verbose)
+  ;  ALL ; all messages
+
+  ; default_level INFO
+
+  ; You may override default_level by assigning a logging level
+  ; to the desired module name. Module names can be found in two ways:
+  ;
+  ; Run:
+  ;   nfd --modules
+  ;
+  ; Or look for NFD_LOG_INIT(<module name>) statements in .cpp files
+  ;
+  ; Example module-level settings:
+  ;
+  ; FibManager DEBUG
+  ; Forwarder INFO
+}
+
 ; The face_system section defines what faces and channels are created.
 face_system
 {